Prospecting



Dec. 26, 1950 G. HERZOG 2,535,056

PROSPECTING Filed March 9, 194a 2 Sheets-Sheet 2 FIG. 2.

ANT/COUNTERS COSMIC RAYS L- --T0 CONDENSER 23 HIGH VOLTAGE W SUPPLY l6TUBULAR CD9?) (5 1 ANODE WIRE PROSPECT/N6 (I COUNERS/ V I L- 7'0CONDENSER 22 LEAD SHIELD EART H SURFACE 1% .fCOSM/C RADMT/ON EA RThSURF/1 C E i INVENTOR.

GL'RHARD HERZOG A TTORNEY Patented Dec. 26, 1950 PROSPECTING=GerhardaHerzoe, Houston, ZHex.,1:assignor-;toflhe vTexas,(lompanyy,NewYork,,N. '.;acor-noration .of. Delaware Application lvlarch ii),1948,=Seria 1 No.=-1S';843

L19. lClaims. (Cl. 250.:83.'6)

This invention. is, (concerned :with wprospecting and particularly:{with r the discovery of mineral depositsbytimeasuringdifierences ingammaaray intensity :in -.:the me-ighborhood of :the @deposit. Theinventionzprovides ;methods and apparatus forcincreasingthe. accuracy.of :the measurements, toil-1e ,end'gthatuzthe probability of elocating':minanal depositsithrough surveys, of .the'gradioactivity particularly.gamma. radiation, ,of thesurroundine or; covering crooks,.issubstantially increased.

' .As .1 disclosed an zcoeneneine appl cation SerialNo;.;13,82,5.iile,dMarsha), 1948, it. is theoretically inmracticablei to;llocate;-buried; ,ore;'depos-its,-,1even thouehztheserbhighlyradioactive,int SuPVQYS-iiOf radioactivity in the-:neighborhoodofthe .deposits, exceptin' :those' cases. ,in-which the: OVBIbIlP-r den issoqth-in that itdoesnot absorb the radiations from :the ore-deposit tothe extent that significant variations-inray intensity dueto thepresence: of ithe deposit; are 1 obscured. frhe matural iy, t ioactive:elemonts, whichinclude :the uranium ,=thoriumand;.;actinium, familiesand ptassium, emit alpha, beta and gamma -rrays in various proportions:and -;with sliflerent energies. The alpha and beta radiations-shave-:but slight penetrating :PDWI. The .gaenetrating power of thegamma-raysishig-heritheabsorption o-fthese 'iiays: by; any. :surroundineimedium.is governed: by the: exponentialiaew. Assuming a, reasonable ab,-sorption coeiiioientof perzcm, iorthe medium, sayrock, 1athicknessgofaii centimeters will reduce .theintensity of the hardest;(i.-,e. most penetrating) gamm-arays, of thorium .C ;by at factor: oi1.0,; 46wcentimetersreduces the intensity-by a factor-of ,100; :69'.centimeters byaiiactoraof' 1000;-.92 centimetersbyza.factor-,ofililflllO; 1-15 centimeterszbysa factor of '100,000,-etc.Substantiallyallrocks .are radioactive to some extent, and even .withthe, most sensitiveoof available vdetectors, an overburden of afewmeters reduces the intensity of the gamma radiation to the point whereit becomes impossible to .recognize ,the excess activityoriginating intheore body over the activity originating in the ,overburden.vltlevertl'ieless,v as ,describedand claimed in the aforementionedapplication, it is possible to recognizevand locate ore. bodiesthroughcover much. inexcess of .onemeter, by means .of variations from point topoint in the intensity of amma raysoriginatingin theoverburden itself.Thus...it.has.been discovered that .ore bodies may be revealed throughdetection of .variationsnof gamma radiation vbetween points so iarremoved from the ore body. itselfsas to be, forall. prac-. ticalpurposesflheyond tithe range ..at which ,sig-

2 nificant differences ofdntensity .,of r.gamma raysoniginatinginzthehre body are detectable, ,IfIn

short, .a gamma..rayedetector,,if ;it,=is sufliciently,

efficient sand is employed for; a silfiicient iy n time at each :nointwherezameasurement :of

myzintensity; is measimed, may .be ,iisedtisooversignificantidiiferencesin the gamma. emission iromavgirl-Lirality.of locations in: the,

overburden which. indirectly, ',-reveal .:the pr enoeof g-IZhBJrO e.lbody,

. 'Iihe inventionsof the:,-af.orementioned @applicaa ion.,'.is;pnotrestricted -'GO :the discovery of ore bodies ofthetradioacti-ve;elements. In;fact,:it finds its major application inprlospecting iOI'JDXE bodies i of unoneradioacti-iie ;,metals such asiron,

copper, :iead, tin,;zinc, .:etc. ltishas been discov-.

ered ithQiTfllEQiC-QUEDW-Tflfikih whichsuchsdepos its occur generallyshows-significant, deviations in; gamma rayintensity;tromswhichthe:existence and donationv oi the adenositomay be determined.In other awordsithendepositshave ,iaintly radioactive aurastwhichgifspmperly detected, act .as markers-norv the idepositsgwhether or not thedepcsits-rt'hemselwmare radioactive. 7

is no 1 substantial aicontent of commercial minera-is, particularlytheynon-radioactive metallic minerals, are; ingeneralz radioactive tosome extent, and may-show-.rad ioactivity equal to severalmicro-micrograms of ra :per gram of rock. It therelatively slight-'di-iierencesin this radio'- activity of the-overburdenor I country-rock;particularly gamma :ray intensity, at---spaced-points in proximity to anore body which indicate the presence of the laitter. As 'i'nd-icatedabove; the

determination-of these differences in-reasonaible observation timesan'd"with detectors of -reason able size, requires the-use-of -detectors"having a high gamma ray-countingefficiency, several times the efficiency ofthe convent-iona-l'Geiger Mueller counter-consisting of a tubularcathode arounda wire anode. Such conventional ooun tersihave anefficiency ofnot toexceedabout' /2% -i.-e.--they detect onthe averageonly one 3 tions in background, caused for example by cosmic rays.

A detector which is sensitive to gamma rays is also sensitive to cosmicrays, and hence these contribute to the variable background. Cosmic raysoriginate in the universe and at present it is accepted that theoriginal cosmic rays consist of protons. During the travel of theseparticles through the atmosphere they produce secondary rays, namelymesotrons, electrons, and gamma rays, and at the earth's surface amixture of all of the foregoing constituents is found and directedgenerally downward. The gamma ray constituents of the cosmic radiationmay be stopped to some degree by interposing a shield of lead or thelike, the background due to this radiation being correspondinglyreduced. The electrons and mesotrons of the cosmic radiation are verypenetrating and the thickness of shielding required to keep these fromaffecting the gamma ray detector is prohibitive.

- Cosmic radiations vary with latitude and altitude. There are alsodiurnal variations, i. e. as between day and night, and irregularvariations, probably attributable to magnetic storms. The variations dueto altitude are the greatest and may be quite large when a survey ismade with an airborne gamma ray detector or when the terrain on whichthe survey is made is mountainousa common condition in mining districts.All contribute to variations in the background of a gamma ray detectorand may obscure the slight differences of gamma ray intensity upon whichprospecting depends.

In co-pending application Serial No. 13,847, filed March 9, 1948, it isproposed to minimize the elfect of cosmic ray background by employingtwo detectors, one of which (the prospecting detector) detects a higherproportion of gamma rays than the other (called the background counter)but both of which have a relatively high efficiency for the detection ofbackground radiation. The two detectors are placed near each other sothat they detect gamma radiation from the same general locality in theearth and also are subject to 4 approximately the samebackground. Theoutputs of the two detectors are then subtracted from each otherelectrically, preferably after integration and amplification and thedifference is measured. In this fashion the difference in averageintensities determined by the detectors over a selected time period isobtained, and this difference is representative of gamma intensity. Thetwo detectors may be made to detect difierent proportions of the gammarays present by shielding one of the counters or by constructing one sothat it has a. lower efliciency for gamma rays than the other or both.

In accordance with the instant invention a somewhat similar result isobtained but the method and apparatus are different, for instead ofsubtracting averages from each other a process of anti-coincidenceinvolvin individual rays is employed. Thus the invention contemplates aprocess of prospecting involving the detection of intensity of gammaradiation from earth with a first detector sensitive to both gamma andcosmic radiation in the presence of cosmic rays, at least one additionaldetector sensitive to cosmic rays being interposed in the path of thecosmic rays passing to the first detector, and observing only thoseindividual rays which are detected by the first detector alone. Thepractice of the invention involves connecting the detectors in ananticolncidence circuit provided with an indicator 4 which responds onlyto the counts which are not common to the two detectors. In this mannerthe cosmic ray background i substantially reduced and the accuracy ofdetermination of gamma ray intensity from the ground is increased.

In the preferred practice of the invention the two detectors, which maybe either single counters or banks of counters, are'disposed adjacenteach other and so oriented that a cosmic ray will pass through both. Asuitable shield for gamma radiation i disposed between them. One of thedetectors is disposed toward the source of gamma radiation to bemeasured, i. e. toward the earth. The other is disposed toward thesource of cosmic radiation, 1. e. outward. The shield thus serves twofunctions: it stops gamma rays originating in the earth from reachingthe oute or anti-coincidence detector and it stops some gamma ray andother constituents of the cosmic radiation from affecting the inner orprospecting detector. Thus the response of the anticoincidence detectorto the gamma radiation from the earth is reduced while the response ofthe prospecting detector to the less penetrating part of the cosmicradiation is also reduced.

The shielding of the anti-coincidence detector, although preferred, isnot essential to the practice of the invention. All known detectors ofthe counter type, i. e. those which are directly responsive toindividual rays, have a high efficiency for the particles in cosmicradiation and a relatively low efficiency for gamma radiation.Consequently, the probability that a given cosmic ray will trip bothcounters is high, while the probability that the same gamma ray from theearth or from cosmic rays will trip both counters is low, even thoughthe shield be omitted. other words, the chance of gamma ray coincidenceis low and the chance of cosmic ray particle coincidence is high, thenet result being that most of the gamma rays detected by the prospectincounter will pass through the anti-coincidence circuit to the indicator,while the cosmic ray background will be suppressed, thus increasingcontrast.

, This contrast may be increased further by employing as theanti-coincidence counter one which has a lower efficiency for gamma raysthan the prospecting counter has while retaining high efiiciency forcosmic rays in both counters. This difference in efiiciency in detectionof gamma rays may be attained in several ways, viz:

1. By employing an anti-coincidence detector which has a smaller cathodearea per unit active volume than the prospecting detector.

2. By making the cathode of the anti-coincidence detector of material oflow atomic number, say aluminum or beryllium, and making the cathode ofthe other detector of material having a higher atomic number, saysilver, tantalum or tungsten.

These and other aspects of the invention will be more thoroughlyunderstood in the light of the following detailed description, taken inconjunction with the accompanying drawing in which:

Fig. 1 is a diagrammatic illustration of a practice of the inventionemploying an anti-coincidence circuit;

Fig. 2is a diagram showing a preferred form of detector set-up employinga plurality of prospecting and anti-counters; and

Fig. 3 is a diagram illustrating still another set-up of detectors,employing a single prospecting counter and a plurality of anti-counters.

.5' Ina prospecting:surveyotgammaxrayintensity at anumbenof pointszoned-n. 1.01" over thaearth. the apparatus of.Fig. 1 .is .so udisposedthatlits anti-counter 1.0 ;is sex-posed :tdirectly 15110; cosmic.

radiation zahut shielded from gamma .:-radiation originating in theearth byxa sineldz l-"l :of; leader the like which overlies 1a:prospecting counter 12 that:receivesithezgamma radiationiromrtheieurthand is notshielded. The connterszemnloyedrmay be of any type sensitiveto gamma radiation, but in. order .to reduce the ":time mequired r'iormeasurement at each point. the prospectin counter should have i high;.=emoiency 5:10! ,gamnia ra'dition. For purposes .of illustration.:however. the .two counters of Fig. 1.1 zare zshownsas :;self.-quenching 'GeigereMueller' .oounters .;having an anode. 1:3 rimmingthrougheazrtuhularnathode I4, both :elementshei-ng enclosedxzinansenvelope 1-5 of glass or' the iikez'wh-ich :retainszassuitable ;atemosphere aroundia-nodeand. cathode.

Generally speaking, the :higher the ..efiioienoy of the prospecting:counterdn.ztermszofagamma ray detection .the morezaccurateuisthaapparatus for determining the Significant diiierences 1in gamma rayintensity of gthe earth, :and the shorter the required .observationtime. Thus counters. of the .typevdesoribed and. claimedin U, .S.Patent1No.'2;39fZ.,071.granted March .19, 1946 are preferred.fiuohzcounters provide a plurality of .cathode...disks.spacedfrom-.-.each other within an envelope :with :their major .suriacesapproximately 1 parallel.randz provided with holes through which one .orunore :anode wires -;are stretched and extend :out of -..contact 1withthe plates and transverse :to ltheirvmajor..-surfaces.

crystal .ty-peof counter .may a-lsd'he -used,..and

required observation time reduced: byincreasing size or .employingapluralitv iof theselelectricaily connected in: parallel.

To return to Fig. 1, the prospecting rcounten,

and the ninth-counter are nboth :connected i110 5.2. high voltagesupply. of direct current. :Both anodes are connected .to-the positive.side oithis voltage supply and home-cathodes; are-.lconnected to thenegative sidecof'the supply,rhut 1a ouenching resistance. I1 .:isinserted. inatheiline.-.between the voltage supply .andi-thevanodezzof:theprospecting counter and a quenchingeresistanceil8;is3 in.- sertedinthe line between .the; cathode of .the anti-counter and. thenegativerside of '5:the*=il0lb age supply.

Both counters are connected 7120211116 input of a mixing circuit 43 ofnn-anti-noincidence; type, the output of the circuit :being. connected:torra pulse indicator Gii'lGOOhdGlfjlfllQl theequivalent. The purposeof i the mixingcircuits toipass only those counts from thevprospecting.counter which are not accompanied: :by simultaneousdischarges in the anti-counter. #Anti-coincidence circuits are not per.se novel 'and 'have been disclosed by G. Herzog, in Review "ofScientific "instruments vol. 11, p.84. (Hemp-and.by :V.; Sarabhai,.inPhysical Review, vol. --65, 'p. 1-203 5 (1944) and a number .of' suchcircuits are known.

The "circuit disclosed in Fig. :1 @has .;a,:1high efliciencyin.:supp1'-eS.Sing .undesi-redpulses the practiceroi l the invention..rltsoperiationis based.

uponthe connectionof the counters to the mixing .-.circuit :in; suchfashion that. the anti-counter gives aim-pulses which :are opposite insign to those. or the prospectin .counter. The sumtwo -..simultaneous;pulses .;from the two counters is thus, made such that it .does not tripthe output stage .of: the anti-coincidence .;circuitand :send .a pulseto the .recorder.

.Inthecconnection of thestwo counters of Fig. 1 1120 their '.,highvoltagesupply, the; respective quenching. resistors l1, I8 areconnectedinqoppositmsidesof "the .supply. It willalso be observed: that :acondenser 22 in the :mixing circult -.is connected on. its input sidebetweenthe prospecting counter; and its quenching resistor l1 :and thata condenser 23 in :the mixing- .cir-- ouitlis connected on. itsinputside between the anti-counter and its quenching resistor 1:8.

Thusrdue. totheposition of the quenching resistors, -:the :pulsesentering .the mixing circuit through the condensers are .of oppositesign.

Themixing circuit includes a pair of tetrode vacuum .tubes 24,;25.connected in parallel, and a.;trigger tube .26. The control grid .of thefirst tetrode is. connected to the prospecting counter throughtheoondenser .22. and the control grid of-the. .other.tetrode '25 isconnected to theanticounter through the. other condenser 23.

The control grid of the trigger tube is coupled through. a condenser '27.to the plate of thetwo tetrodes, and the recorder .is connected acrossthe platev and cathode, i. -e.- across the output-of the trigger tube.

A conventional voltage divider. network (only partially indicated onFig. 1) is employed to obtain the required potentials at various "pointsintheirmixing circuit.

-In operation, the condenser22 receives "a negative pulse from theprospecting counter thus increasing the inner resistance of the slightlypositive biased tetrode 24, and thus originating a'positive pulse on thecondenser 21. This condenser feeds :the grid of the trigger tube in theusualmanner and activates the'pulse recorder.

Thesbias .011 the .other'tetrode .25 is-made sufficiently negative thatthe positive anti pulseson' the condenser 23 .from the anti-countergives a negative pulse at the condenser 2'!" which feeds the triggertube.

z-Theflpulses from the tube 25 arrive at the condenser with a negativepolarity. The trigger tube 26 is biased beyond cutoff, so that suchnegative pulses cannot afiecttheplate current through this trigger tubeand hence will not be transmitted to the recorder. Thus the anticounteralone does not. aiTect the'recorder. .In'

short, the only "pulses which '"are counted are those whichoriginateinthe prospecting counter atxtimes-when no pulse originates in theanticounter and... hence are representative 0f radiation received by theprospecting counter from the earth, and not ofzcosmic rays which passthrough both counters.

Typical :values for voltages, resistances, and

capacitancesfor various parts ofthe circuit'are' shown .on Fig. 1.

It will. lee-understood that a varietyv of types 0 antiecoincidencecircuits may be employed. in the practice of.:the'invention, that i=ofFig. 1 being preferred oversome others merely by reason of itssimplicity. v :i'lnzsome cases, in order to secure adequateconntingirates sot-hat cbservationszmay :be made in reasonable time, itis desirable to employ banks or bundles of counters. Such an arrangementis shown in Fig. 2 wherein a bank of anti-counters is disposed above alead shield whereas a bank of prospecting counters is disposed below theshield toward the earth. In other words, the anticounters are shieldedfrom the gamma rays originating in the earth and not from the cosmicradiation, whereas the opposite is the case for the prospectingcounters. The bank of prospecting counters acts as a single unit and thecounters are connected in parallel with each other to a high voltagesupply and to an anti-coincidence mixing circuit in the same fashion asthe apparatus of Fig. 1. Similarly, the anti-counters of the other bankare connected in parallel (to act as a single unit) with the highvoltage supply and to the anti-coincidence mixing circuit. If a greatnumber of counters is used in each bank, it is sometimes advantageous toconnect each counter to a separate preamplifier and to then feed thepreamplified pulses into the mixing circuit.

In some cases there may be advantages to shielding the prospectingcounter or counters in all directions except that from which thesignificant gamma radiation of the earth is received, or interposing aplurality of anti-counters above and around the prospecting counters soas to intercept cosmic radiation from the side as well as from above.Both of these situations are illustrated in Fig. 3 wherein a. singleprospecting counter is exposed to earth radiation but its bank ofanti-counters is arranged to intercept cosmic radiation arriving at avariety of angles, the anti-counters being shielded from the earthsgamma radiation by a lead shield that is roughly U-shaped in section,

At the outset it was indicated that a portion of the cosmic radiation isso penetrating that shielding against it is impractical. However, theless penetrating portion, including the gamma ray component of thecosmic radiation may be kept from both the prospecting counters andanticounters by means of an overshield (Fig. 3) interposed in the pathof the cosmic rays to the anti-counters, and the use of such a shieldfor the counters is within the concept of the instant invention.

The invention finds its major field of application in the location ofanomalies in the gamma ray intensity over an area in the neighborhood ofan ore body. Gamma radiation from the ore body itself seldom contributessubstantially to the observed anomaly, which generally is attributableto small diiferences in the intensity of gamma radiation originating inthe substantially barren overburden or country rock. Ore bodies to besought through the practice of the invention need not be and usually arenot those of the radioactive metals. Thus the invention is applicable toprospecting for ore bodies and mineral deposits of base metals such asiron, molybdenum, tungsten, copper, zinc, lead, tin, etc., and ofprecious metals such as gold, silver and platinum. Anomalies may beobserved in both igneous and sedimentary rocks and in connection withplacer deposits as well as those occurring in solid rock.

Surveys may be made with airborne equipment at elevations of severalhundred feet above the earth and major anomalies may be so locatedrapidly and economically. Detailed surveys may be made on the surface ofthe earth with the radiation detection equipment carried by man, animalor vehicle. Detailed surveys may also be made underground in any type ofworking. in-

eluding shafts, winzes, and raises and drifts, cross cuts, adits andtunnels as well as in bore holes drilled at any angle. If the detectionis sufllciently efficient in terms of gamma radiation, it is notnecessary that the equipment be stationary while observations are made,and significant anomalies may be detected while flying the equipment athigh speeds or while carrying it at substantial velocity along atraverse with automotive equipment.

To consider a simple case, an area to be prospected is surveyed as arectangular grid, each intersection on the grid being an observationpoint. At each such point the apparatus is set up and the gamma rayintensity from the earth is measured in terms of counts or pulses perunit of time, say minutes or seconds. The anti-counter and theanti-coincidence circuit are employed to reduce background in each case.The observation period at each point is sufficiently long to obtainsignificant differences in gamma ray intensity. With conventional singlecounters of small size and efiiciency, this time may be too long forpractical purposes; consequently the use of high efficiency counters isrecommended, in size or number sufficient that the observation time bewithin practicable limits. The intensities at the several points thushaving been obtained, the intensities or differences in intensity areplotted and like values joined by contour lines. Study of the completedmap may disclose anomalous highs" or lows which reveal the presence ofan ore body. Instead of directly observing the counts per unit time anintegrating circuit can be used in combination with a recorder whichgives the pulse rate,

I claim:

1. In prospecting, involving the detection of the intensity of gammaradiation from the earth with a first radiation detector in the presenceof cosmic rays to which it is sensitive, the improvement which comprisesinterposing at least one additional detector sensitive to cosmic rays inthe path of the cosmic rays passing to the first detector, and observingonly those rays which are detected by the first detector alone. 2. Inprospecting, involving the detection of the intensity of gamma radiationfrom the earth with a first radiation detector in the presence of cosmicrays to which it is sensitive, the improvement which comprisesinterposing at least one additional detector sensitive to cosmic rays inthe path of the cosmic rays passing to the first detector, interceptingthe gamma radiation from the earth to the additional detector byinterposing a shield between the two detectors, and observing only thoserays which are detected by the first detector alone.

3. In prospecting, involving the detection oh the intensity of gammaradiation from the earth with a, first radiation detector in thepresence of cosmic rays to which it is sensitive, the improvement whichcomprises interposing at least one additional detector sensitive tocosmic rays in the path of the cosmic rays passing to the firstdetector, detecting the gamma rays from the earth in the first detectorwith higher efficiency than in the second detector, while detecting thecosmic rays with still higher efiiciency in both detectors, andobserving only those rays which are detected by the first detectoralone.

4. In prospecting, involving the detection of the intensity of gammaradiation from the earth with a first radiation detector in the presenceof cosmic rays to which it is sensitive, the improvement which comprisesinterposing-at least one :additionaldetector: sensitivevto cosmic raysinvthe. path. of. the cosmia-raysr assing. to the first detector, andobservingrby antiecoincidence only thoseindividual rays whicharedetected' by the first detector, alone.

5. In prospecting apparatus, the'combination which comprisesa-ra'diation detector sensitive to both gamma and cosmic rays, a seconddetector sensitive to-cosmic rays disposed adjacent the first detectorin the path of cosmic rays passing thereto, a pulse indicator, and ananti-coincidence circuit connected to the detectors and to the indicatorandso arranged that radiation simultaneously detected by both detectors'isz'suppressed while only that. detected by the first-detectoristreg-istered by the indicator.

6. In prospectingv apparatus, the combination which comprises aradiation detector sensitive to both-gamma and cosmic rays and disposedtoward the earth, a second detector sensitive to cosmic rays disposedadjacent thefirst detector i-n the-path of' cosmic-rayspassing-"thereto,a pulse indicator, and an anti-coincidence circuit connected to thedetectors and to the indicator and so arranged that radiationsimultaneously detected by both detectors is suppressed while only thatdetected by the first detector is registered by the indicator.

7. In prospecting apparatus, the combination which comprises a firstradiation detector sensitive to both gamma and cosmic rays, a pluralityof additional detectors sensitive to cosmic rays disposed adjacent thefirst detector in the paths of cosmic rays passing thereto, a pulseindicator, and an anti-coincidence circuit connected to the detectorsand to the indicator and so arranged that radiation simultaneouslydetected by the first detector and one of the plurality of additionaldetectors is suppressed while that detected by the first detector aloneis registered by the indicator.

8. In prospecting apparatus, the combination which comprises a firstradiation detector sensitive to both gamma and cosmic rays, a seconddetector sensitive to cosmic rays and having a lower efficiency forgamma rays than the first detector disposed adjacent the first detectorin the path of cosmic rays passing thereto, a pulse indicator, and ananti-coincidence circuit connected to the detectors and to the indicatorand so arranged that radiation simultaneously detected by both detectorsis suppressed while that detected by the first detector alone isregistered by the indicator.

9. In prospecting apparatus, the combination which comprises a radiationdetector sensitive to both gamma and cosmic rays, a second detectorsensitive to cosmic rays disposed adjacent the first detector in thepath of cosmic rays passing thereto, a shield capable of absorbing gammaradiation disposed between the detectors, a pulse indicator, and ananti-coincidence circuit connected to the detectors and to the indicatorand so arranged that radiation simultaneously detected by both detectorsis suppressed while that detected by the first detector alone isregistered by the indicator.

10. In prospecting apparatus, the combination which comprises a gammaradiation detector, a shield for gamma rays disposed between thedetector and the earth, a second detector disposed below the first andunshielded from said gamma radiations, a pulse indicator, and ananticoincidence circuit connected to the two detectors 10 andt to theindicator'and so "arranged thatradia; tion-asimultaneouslyy detected by1 both. detectors is suppressedwhileethat detected by the seconddetector alone is registered by the indicator;

11'; Apparatus according to claim 10 provided with. a; shield above thefirst detectorfor shieldingit :at least partially, against cosmicradiation.

12... In measuring the -intensity of gamma-radiations from-can earth:surfacewin thepresencaof cosmic: rays, the improvement which comprisessimultaneously employing two gamma; ray counters; one 'disposedrtowardthe earthsurface andrtlie' other adjacent but toward theoriginvoftheticosmic' rays, WithPa; shieldifor gammaradiation: disposed:betweezrthe counters and indicatonly the raysxthat are detected-by; thefirst counter" alone;

1'3: In.measuring'the'intensity of gamma :radiationif-romiarr earthsurta'cel ini the' presence 0f 20 cosmic-rays; thewimprovement whichcomprises simultaneouslyemploying two-'gamma' ray counters, one disposedtowardi'the earth surface-:and the second: adjacent'rthexfirst buttowardutheprigin of the cosmic rays, shielding the second from gammaradiation originating in the earth, shielding both counters from thegamma radiation component of the cosmic rays, and indicating only therays that are detected by the first counter alone.

14. In prospecting apparatus, the combination which comprises a firstradiation detector sensitive to both gamma and cosmic rays, a seconddetector sensitive to cosmic rays disposed adjacent the first detectorin the path of cosmic rays passing thereto, a shield capable ofabsorbing gamma radiation disposed between the detectors. a secondshield capable of absorbing gamma ray components of the cosmic raysdisposed between the second detector and the source of the cosmic rays,a pulse indicator, and an anti-coincidence circuit connected to thedetectors and to the indicator and so arranged that radiationsimultaneously detected by both detectors is suppressed while thatdetected by the first detector alone is registered by the indicator.

15. In prospecting, involving the detection of the intensity of gammaradiation from the earth in the presence of cosmic rays with detectorsof the anode-cathode type sensitive to such rays, the improvement whichcomprises subjecting one such detector to gamma radiation from theearth, interposing another such detector sensitive to cosmic rays in thepath of the cosmic rays passing to the first detector, said seconddetector having a smaller cathode area per unit active volume than thefirst detector, and observing only those rays which are detected by thefirst detector alone.

16. In prospecting, involving the detection of the intensity of gammaradiation from the earth in the presence of cosmic rays with detectorsof the anode-cathode type sensitive to such rays, the improvement whichcomprises subjecting one such detector to gamma radiation from theearth, inter-posing another such detector sensitive to cosmic rays inthe path of the cosmic rays passing to the first detector, the firstdetector having a cathode comprising a material of high atomic numberand the second detector having a cathode comprising a material of 10watomic number, and observing only those rays which are detected by thefirst detector alone.

17. In prospecting apparatus, the combination which comprises a firstseries of radiation detectors sensitive to both gamma and cosmic rays,

a second series of detectors sensitive to cosmic rays, a second seriesof detectors sensitive to cosmic rays and adjacent the first series ofdetectors in the paths of cosmic rays passing thereto, a pulseindicator, and an anti-coincidence circuit connected to the detectors ofeach series and the indicator and so arranged that radiationsimultaneously detected by a detector of the first series and a detectorof the'second series is suppressed while radiation detected only bydetectors of the first series is registered by the indicator.

18. In prospecting apparatus, the combination which comprises a firstseries of radiation detectors sensitive to both gamma and cosmic rays, asecond series of detectors sensitive to cosmic rays adjacent the firstseries of detectors and aligned with respect thereto so that a cosmicray passing through one of the first series of detectors must passthrough one of the second series of detectors, recording means, and ananti-coincidence circuit connected to the detectors of each series andsaid recording means and so arranged that radiation simultaneouslydetected by a de- 12 tector of the first series and a detector of thesecond series is suppressed while radiation detected only by detectorsof the first series is registered by said recording means.

19. The apparatus according to claim 18 wherein a gamma ray shield isprovided between the two series of detectors. GERHARD I-IERZOG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,316,361 Piety Apr. 13, 19432,397,071 Hare Mar. 19, 1946 OTHER REFERENCES Herzog, Review ofScientific Instruments, vol. 11, pp. 84 and 85.

Korfi, Electron and Nuclear Counters, D. Van Nostrand, Apr. 1946, pp.166-468 and 170.

